It is known in the art to use inks, compositions or layers containing magnetic or magnetizable particles or pigments, particularly also magnetic optically variable pigments, for the production of security elements, e.g. in the field of security documents. Coatings or layers comprising oriented magnetic or magnetizable particles are disclosed for example in U.S. Pat. No. 2,570,856; U.S. Pat. No. 3,676,273; U.S. Pat. No. 3,791,864; U.S. Pat. No. 5,630,877 and U.S. Pat. No. 5,364,689. Coatings or layers comprising oriented magnetic color-shifting pigment particles, resulting in particularly appealing optical effects, useful for the protection of security documents, have been disclosed in WO 2002/090002 A2 and WO 2005/002866 A1.
Security features, e.g. for security documents, can generally be classified into “covert” security features one the one hand, and “overt” security features on the other hand. The protection provided by covert security features relies on the concept that such features are difficult to detect, typically requiring specialized equipment and knowledge for detection, whereas “overt” security features rely on the concept of being easily detectable with the unaided human senses, e.g. such features may be visible and/or detectable via the tactile senses while still being difficult to produce and/or to copy. However, the effectiveness of overt security features depends to a great extent on their easy recognition as a security feature, because most users, and particularly those having no prior knowledge of the security features of a therewith secured document or item, will only then actually perform a security check based on said security feature if they have actual knowledge of their existence and nature.
A particularly striking optical effect can be achieved if a security feature changes its appearance in view to a change in viewing conditions, such as the viewing angle. Such an effect can e.g. by obtained by dynamic appearance-changing optical devices (DACODs), such as concave, respectively convex Fresnel type reflecting surfaces relying on oriented pigment particles in a hardened coating layer, as disclosed in EP 1 710 756 A1. This document describes one way to obtain a printed image that contains pigment particles or flakes having magnetic properties by aligning the pigment particles in a magnetic field. The pigment particles or flakes, after their alignment in a magnetic field, show a Fresnel structure arrangement, such as a Fresnel reflector. By tilting the image and thereby changing the direction of reflection towards a viewer, the area showing the greatest reflection to the viewer moves according to the alignment of the flakes or pigment particles.
While the Fresnel type reflecting surfaces are flat, they provide the appearance of a concave or convex reflecting hemisphere. Said Fresnel type reflecting surfaces can be produced by exposing a wet coating layer comprising non-isotropically reflecting magnetic or magnetizable pigment particles to the magnetic field of a single dipole magnet, wherein the latter is disposed above for concave effect (FIG. 2C bottom), respectively below the plane of the coating layer for convex effect (FIG. 2C top), as illustrated in FIG. 7B of EP 1 710 756 A1 for a convex orientation. The so-oriented pigment particles are consequently fixed in position and orientation by hardening the coating layer.
One example of such a structure is the so-called “rolling bar” effect, as disclosed in US 2005/0106367 and U.S. Pat. No. 7,047,883. A “rolling bar” feature is based on pigment particles orientation imitating a curved surface across the coating and provides the optical illusion of movement to images comprised of oriented pigment particles. The observer sees a specular reflection zone which moves away or towards the observer as the image is tilted. A so-called positive rolling bar comprises pigment particles oriented in a concave fashion (FIG. 2B) and follows a positively curved surface; a positive rolling bar moves with the rotation sense of tilting. A so-called negative rolling bar comprises pigment particles oriented in a convex fashion (FIGS. 1 and 2A) and follows a negatively curved surface; a negative rolling bar moves against the rotation sense of tilting. A hardened coating comprising pigment particles having an orientation following a concave curvature (positive curve orientation, shows a visual effect characterized by an upward movement of the rolling bar (positive rolling bar) when the support is tilted backwards. The concave curvature refers to the curvature as seen by an observer viewing the hardened coating from the side of the support carrying the hardened coating (FIG. 2B). A hardened coating comprising pigment particles having an orientation following a convex curvature (negative curve orientation, FIG. 2A) shows a visual effect characterized by a downward movement of the rolling bar (negative rolling bar) when the support carrying the hardened coating is tilted backwards (i.e. the top of the support moves away from the observer while the bottom of the support moves towards from the observer) (FIG. 1). This effect is nowadays utilized for a number of security elements on banknotes, such as on the “5” of the 5 Euro banknote or the “100” of the 100 Rand banknote of South Africa.
For optical effect layers printed on a substrate, negative rolling bar features (orientation of the pigment particles (PP) in a convex fashion, curve (FIG. 1 and FIG. 2A) are produced by exposing a wet and not yet hardened coating layer to the magnetic field of a magnet located on the opposite side of the substrate to the coating layer (FIG. 2C top and FIG. 3), whereas positive rolling bar features (orientation of the pigment particles (PP) in a concave fashion, curve (FIG. 2B) are produced by exposing a wet and not yet hardened coating layer to the magnetic field of a magnet located on the same side of the substrate as the coating layer (FIG. 2C bottom and FIG. 4A left). Examples of positive and negative rolling bar features and combinations thereof, i.e. double rolling bar features and triple rolling bar features, have been disclosed in US 2005/0106367 and in WO 2012/104098 A1, respectively. For positive rolling bar features wherein magnet is facing the still wet and not yet hardened coating layer, a simultaneous curing of the coating layer with an irradiation source, such as for example a UV irradiation source, for fixing the orientation of the pigment particles within the coating layer is prevented thus allowing said curing only after the removal of the coating layer from the magnet.
U.S. Pat. No. 2,829,862 teaches the importance of the viscoelastic properties of the carrier material for preventing reorientation of the magnetic or magnetizable pigment particles after the removal of the external magnet. Maintaining the coating composition comprising the magnetic or magnetizable pigment particles or flakes within the magnetic field during the curing process preserves the orientation of the magnetic or magnetizable pigment particles. Examples of such processes are disclosed for example in WO 2012/038531 A1, EP 2433798 A1 and US 2005/0106367. In all these examples, the external magnetic device is located on the side of the substrate opposite to the side carrying the coating composition and the curing process is triggered by an irradiation source located on the side of the substrate carrying the coating composition.
It is known in the art that when a coating or ink composition is cured using a UV-VIS irradiation source, the exposure conditions of the coating or ink composition to the irradiation source are crucial to obtain a through-cure and fast curing of the composition. Preferably the irradiation source is located directly facing the to-be-cured coating or ink composition.
JP 06122848 discloses a printing method for intaglio printing wherein an intaglio ink is cured with electron beam from the backside of the substrate immediately after the ink application. Whereas curing with the use of electron beam allows curing through optically opaque material, however, said mechanism requires shielding of the apparatus with heavy metal parts thus leading to cumbersome equipments and being highly demanding in term of safety. Moreover, electron beam curing is strongly inhibited by atmosphere such that efficient curing disadvantageously need inert atmosphere.
EP 0338378 A1 discloses a method for producing documents or other articles containing at least one replica of a surface relief diffraction pattern. The method comprise the steps of printing a liquid casting resin on a defined area of a substrate, holding the resin between the substrate and a master of the surface relief pattern and curing it. The type of radiation that is used depends primarily upon the resin formulation and the nature of the substrate material. For substrate made of papers or of other opaque sheet material, electron beam is preferred. For optically transparent sheet material, UV-Vis irradiation may be used.
WO 2005/051675 A1 discloses an apparatus and a method for printing a curable composition to produce diffraction grating on a security product. The composition is cured by using UV-Vis irradiation or electron beam. If the curable composition is applied on a paper substrate and is cured with a UV-Vis irradiation lamp, the lamp is preferably located on or in the means used to form the diffraction grating, i.e. the UV-lamp is located on the front side of the substrate carrying the curable composition. Other examples of holograms prepared by contacting liquid composition with relief structure while simultaneously curing the composition with electron beam from the backside of the substrate have been disclosed e.g. in WO 2000/0534223 A1 or in EP 540450 A1. WO 2012/176126 A1 discloses a method and an apparatus for forming a surface relief microstructure on a paper substrate. The method comprises the steps of applying a composition on the front side of a substrate, contacting at least a portion of the curable composition with surface relief microstructure, and curing the coating composition by using at least one UV-lamp which is arranged on the backside of the paper substrate.
WO 02/090002 A2 discloses a method for producing image coated articles by using magnetic pigments. The method comprises the steps of applying to a substrate a liquid coating comprising non-spherical magnetic pigments dispersed in a pigment vehicle, exposing the liquid coating to a magnetic field and solidifying the coating by exposure to electromagnetic radiation. The solidifying step may be performed with a device comprising a lamp equipped with a photomask such that only parts of the liquid coating are selectively cured, while un-exposed parts of the coating remain liquid. The non-spherical magnetic pigments dispersed in the un-exposed parts of the liquid coating may be re-oriented using a second magnetic field.
Therefore, there remains a need for a process to produce security features displaying a OEL on a substrate, said OEL comprising a plurality magnetic or magnetizable pigment particles oriented in a concave fashion.